This invention relates to a process and apparatus for the removal of undesired gaseous components from a flue gas produced during a combustion process, wherein the flue gas is cooled, optionally subjected to preliminary purification and optionally depleted of SO.sub.2.
The problem of the removal of deleterious gaseous components from combustion gases has gained icnreasing importance. These combustion or flue gases are usually produced by air combustion of hydrogen or carbonaceous fuels, such as oil, coal or natural gas. The removal of any sulfur compounds present, particularly sulfur dioxide, can be advantageously effected with a physical absorbent, after cooling and--if necessary--preliminary purification (especially dust and soot separation and optionally removal of HF and HCl). In this process, almost the entire sulfur dioxide originally contained in the flue gas is scrubbed out. Such a method has been described, for example, in DOS 2,848,721 or DOS 3,237,387.
This conventional process, however, removes only sulfur dioxide. In many instances, however, flue gas contains other components which must not pass into the atmosphere, such as nitrogen oxides in particular.
For the removal of nitrogen oxides, catalytic processes are known wherein NO.sub.2 or NO is reduced in the presence of ammonia in accordance with the following reactions: ##STR1## at high temperatures of between about 300.degree. and 450.degree. C. to N.sub.2 and water which may be safely exhausted into the atmosphere.
In this NO reducing process the catalyst is conventionally arranged in the hot crude flue gas stream before H.sub.2 electric precipitaor and before the SO.sub.2 removal process, i.e. the stream having a high dust content, and/or a high SO.sub.2 and SO.sub.3 content.
In order to free the above gas of NO.sub.x, there are employed honeycomb-shaped catalyst beds housed in containers. The operation in the presence of contaminants, however, results in a relatively brief lifetime of the catalyst. Moreover, another disadvantage of theconventional process for removal of NO.sub.x resides in that in case of peak load, the amount of NH.sub.3 to be added is very dificult to control since, with a change in the amount of flue gas, the temperature in the catalyst bed fluctuates as well, and catalyst selctivity is very sensitive to temperature changes.